Wavelength division multiplexed (WDM) amplifiers amplify optical signals that are composites of multiple wavelength optical signals. WDM optical communications systems relay multi-wavelength composite optical signals through multiple optical amplifiers.
The band over which losses are low in optical fiber transmission circuits (less than approximately 0.3 dB/km) is the band from 1450 nm to 1650 nm. As shown in FIG. 1, a variety of optical fiber amplification devices have been developed for this transmission band.
At present, with the popularity of cellular telephones and the rapid increase in internet use, the demand for telecommunications capacity is expanding explosively. There are global intense research and development efforts for technologies that can increase the information transmission capacity on a single fiber.
Optical wavelength division multiplexing (WDM) technology that uses the broadband characteristics of optical fiber amplifiers having silica erbium-doped fibers (EDF) is critical. The conventional wavelength band is known as both “the 1550 nm band” (1530 to 1560 nm) or “the C band” (conventional-wavelength band).
In addition, EDF optical amplifier equipment for a 1580 nm band (1570 to 1600 nm) “the L band” (longer-wavelength band) has been developed. The competition has become intense in developing a commercial optical fiber telecommunications system that is able to transmit an ultra large capacity (perhaps 1.6 terabit/s) of information by modulating each multiplexed wavelength at 10 Gb/s with about 80 waves in each of the bands for a total composite of 160 waves.
Because there is a capacity of approximately eight THz when C band and L band are combined, when 10 Gb/s transmission signal channels are established with the 2.5 GHz spacing, the overall transmission capacity of 1.6 terabit can be expanded further up to 3.2 Tb/s
  (                    10        ⁢                                  ⁢        Gb        ⁢                  /                ⁢                  s          ⨯                                          ⁢          8                    ⁣              ,                  000          ⁢                                          ⁢          GHz                            25      ⁢                          ⁢      GHz        )
On the other hand, there is demand, for even greater carrying capacity, and so optical fiber amplification devices that have new optical amplification bands, in addition to the current C band and L band, are required.
In FIG. 1, even though GS-TDFA (gain-shifted thulium-doped fluoride-based fiber amplifiers) are being developed for amplification in the S band region from 1490 nm to 1530 nm, GS-TDFA devices have a gain in the region between 1475 and 1510 nm, and thus it may be difficult for them to succeed in the portion of S band extending from 1510 to 1530 nm.
In addition, the 1610 to 1650 nm band is limited to specialty fibers that are either thulium or terbium-doped fluoride-based fibers.
In the optical amplifier devices described above, the optical amplification medium amplifies light through excited emission, which occurs from population inversion of energy levels. There is also Raman fiber amplification, which uses the non-linear effects of fibers. Because Raman fiber amplification makes use of the non-linear effects of fibers, it can produce a gain in any given wavelength band by selecting the wavelength of the stimulating light source. However, there are problems in that the gain per unit length is small, so the optical amplification fibers must placed every several kilometers to every several dozen kilometers within the transmission line.